The devServer is added to the module.exports in the webpack.dev.js. This configures the webpack-dev-server as required. The webpack-dev-server configuration can be set here as well as the command line options, so you as a developer can decide which is better for you.

The output in the module.exports also needs to be configured correctly for the webpack-dev-server to work correctly. If the './' path is used in the path option of the output section, the webpack-dev-server will not start.